Exhaust oxidation

ABSTRACT

The invention relates to a device and a method for reducing emissions in catalytic converter exhaust systems for a vehicle provided with a combustion engine. The invention comprises a control unit adapted to control the air-fuel being fed to the engine so that a high concentration of hydrogen is generated in the exhaust gas during start-up of the engine. Furthermore, secondary air is supplied downstream of the engine during cold starting thereof for forming a gas mixture with the exhaust gas. This gas mixture is oxidized, so that heat energy generated by means of this oxidation is supplied to the catalyst, thereby providing a reduction of the light-off time of the catalyst.

TECHNICAL FIELD

The present invention relates to a device and a method for reducingemissions in catalytic converter exhaust systems far a vehicle providedwith a combustion engine. In particular, the invention relates to adevice and a method by means of which the so-called "light-off" time ofthe catalytic converter can be reduced, while at the same time the rawemissions discharged from the engine to the catalytic converter arereduced.

BACKGROUND OF THE INVENTION

In the field of motor vehicles which are operated by means of combustionengines it is a general requirement that the concentration of harmfulsubstances in the engine's exhaust gas should be as low as possible.These harmful substances are mainly present in the form of unburntresidues of hydrocarbons (HC), oxides of nitrogen (NO_(x)) and carbonmonoxide (CO). In today's motor vehicles equipped with gasoline engines,a purification of the exhaust gas is normally carried out by means of acatalytic converter, or catalyst, arranged in the exhaust system. In themodern so-called three-way catalyst, the major part of theabove-mentioned harmful substances is eliminated by means of variouswell-known catalytic reactions.

Today's catalysts provide a very high degree of purification, i.e. aconversion of harmful exhaust gas components to carbon monoxide andwater. This means that they eliminate a very high quantity of theharmful emissions in the exhaust gas at the appropriate operatingtemperature of the catalyst. However, they suffer from the problem thatthey must be heated for a certain time period in order to reach theoperating temperature at which an optimum degree of purification can beobtained. The so-called "light-off temperature" of the catalyst isapproximately 200-350° C. and can be defined as the temperature at whichthe catalyst provides a 50% degree of purification of a certain harmfulcomponent in the exhaust gases. During the initial warm-up phase of thecatalyst, which is approximately 30-90 seconds, the catalyst cannotoperate in an optimum manner as regards the elimination of the harmfulsubstances in the exhaust gases. Obviously, this constitutes a problemwhich arises during cold starting of a vehicle.

A possible way to reduce the quantity of harmful emissions during saidinitial warm-up phase is to carry out various measures in order toshorten the time taken for the catalyst to reach its light-offtemperature. During a cold start, this can be achieved by generatingincreased heat energy into the exhaust system which subsequently causesthe catalyst to be rapidly heated.

A previously known arrangement for obtaining this reduction in time forthe light-off temperature to be reached is one comprising anelectrically heated catalyst which is arranged upstream of the maincatalyst. However, this arrangement implies certain drawbacks. Firstly,the cost for a heatable catalyst is considerable. Furthermore, theconsumption of electrical energy is relatively high. An additional powersupply such as an extra battery may be required in the vehicle. Also,the durability of the electrically heatable catalyst may constitute aproblem.

Another arrangement, which is disclosed in the journal MotortechnischeZeitschritt, Vol. 55 (1994), No. 4, pages 198-206, "Die Motoren im neuenOpel Omega", Heinz-Ewo Brand at al, comprises means for injectingsecondary air into the exhaust gas. This secondary air is mixed with theexhaust gas in the exhaust port and in a plenum volume immediatelydownstream of the engine's exhaust valves, resulting in an oxidation ofthe mixture consisting of the exhaust gases and the secondary air. Thisoxidation results in a generation of heat energy which is fed to thecatalyst, which consequently will become heated.

This arrangement is based on the fact that, during cold starting, theengine is operated so as to provide a certain stoichiometric excess offuel in the air/fuel mixture which is fed to the engine. The enrichingof the air/fuel ratio to a level which gives a lambda value below λ=1will cause excess hydrogen (H₂), carbon monoxide, (CO) and hydrocarbons(HC) to be generated in the exhaust gases. By reducing λ to a value ofapproximately 0.7 for example, the corresponding amount of hydrogen inthe exhaust gases will then be present in an amount of about 5% byvolume of the exhaust gas.

Such low values of the A parameter can be achieved by altering theengine's control system in such a way that the control output to thefuel injector is arranged to ensure a rich air/fuel mixture during thestart-up phase. Such may occur by increasing the fuel injection timeand/or decreasing the amount of input air to the engine. Further methodsare also available for increasing the amount of hydrogen gas and othercombustible components in the exhaust gas, such as changing fuelinjection or ignition timing, adjusting the timing of the engine valvelifting or even by appying stratified combustion in the combustionchamber.

As previously mentioned, the secondary air is mixed with the exhaustgases, resulting in an oxidation process which is mainly due to thehydrogen which is present in the exhaust gases. The oxidation reactiongenerates a high amount of heat energy which is guided through theexhaust pipe and to the catalyst, which subsequently becomes rapidlyheated.

Although the above-mentioned arrangement results in a reduced time takenuntil the light-off temperature of the catalyst is reached, it suffersfrom the drawback that the performance of the engine Is negativelyaffected. This is due to the fact that the known arrangement cannot beeffectively tuned for optimum engine power and engine torque. As regardsthe tuning of an engine, the design of the engine outlets should alwaysbe considered, so as to provide an optimum volumetric efficiency of theengine. The geometry of the exhaust manifold according to thearrangement described in the above-mentioned article "Die Motoren imneuem Opel Omega" does not allow any such tuning, which is essential ifthe engine's performance is to be optimized.

Another drawback of the arrangement according to said article is thatduring cold starting, little or no consideration is taken to thetemperature of the catalyst. This means that the engine is controlled soas to generate hydrogen when this is not needed, for example when thecatalyst has already reached its light-off temperature. In other words,excess fuel can be supplied to the engine, which results in increasedfuel consumption.

For turbocharged engines the time period to reach optimum performance ofthe catalyst is usually longer than for naturally aspirated engines.This is mainly due to the fact that the turbocharger acts as a heat sinkand reduces the temperature of the exhaust gas before entering thecatalyst. The above-mentioned arrangements for exhaust oxidation have asyet not been applied to turbocharged engines.

Another system for reducing the emissions is known from the documentsWO-A-92/22734 and WO-A-93/07365, which disclose a system in which thehydrogen and oxygen mixture is guided to a separate afterburnercombustion chamber, which is arranged downstream of the exhaust pipe.When the exhaust gas reaches the afterburner combustion chamber it isignited by means of a special ignition device immediately after theengine has first fired. This is achieved by ensuring that theconcentration of hydrogen and oxygen remains within known flammabilitylimits. In order to obtain the required concentrations, the fuel/airmixture is enriched significantly so as to obtain additional hydrogen,whilst additional oxygen is added by means of a supplementary air pump.

Although an improvement is obtained hereby, a severe drawback of thesystem is that an ignition device is required in the afterburner inorder to ignite the gas mixture. Such an ignition device constitutes anextra component which is prone to failure. Moreover, from the consumerpoint of view, this is undesirable due to the resultant extra costinvolved with the more expensive exhaust system and the ensuing costs ofservicing and/or replacing worn-out or faulty afterburner ignitiondevices.

SUMMARY OF THE INVENTION

A main object of the invention is thus to overcome the above-mentioneddisadvantages and to provide an improved device for reducing emissionsin catalytic converter exhaust systems, which device does not involveany significant deterioration in the performance of the engine and whichprovides a significant reduction of the time taken for the light-offtemperature of the catalyst to be reached.

The above-mentioned object is accomplished by a device for reducingemissions in a catalytic converter exhaust system for a combustionengine, said catalytic converter providing reduced emissions after alight-off time has passed during start-up of the engine. Said devicecomprises an exhaust pipe connecting the engine with the catalyticconverter, air/fuel mixture forming means for providing a mixture of airand fuel to the engine, a control unit adapted for controlling theair-fuel mixture forming means so that a high concentration of hydrogenand other combustible gases such as carbon monoxide and hydrocarbons aregenerated in the exhaust gas during start-up of the engine, and airsupply means for supplying secondary air downstream of at least oneengine exhaust valve during cold starting of the engine, for forming agas mixture with the exhaust gas. The engine is provided with aplurality of primary exhaust outlet pipes designed for optimizing anexternal gas exchange system of the engine. Furthermore, said controlunit is adapted to control said generation of hydrogen and the operationof said air supply means, thereby generating an exhaust oxidation insaid gas mixture, so that heat energy generated during the oxidation issupplied to the catalyst, thereby providing a reduction of the light-ofttime of the catalyst.

The invention is particularly useful in engines provided with so-calledprimary exhaust outlet pipes. These pipes correspond in number to thenumber of cylinders in the engine. The primary exhaust outlet pipes (inthe following simply referred to as "primary pipes") are designed aspipe elements which are arranged at the respective cylinder outlets ofthe engine. The primary pipes from the cylinders coincide to form aplenum chamber, to which the exhaust gas from all the cylinders is fed.The exhaust gas is fed through the primary pipes, via the plenumchamber, and a conventional exhaust pipe which leads to the catalyst Thegeometry of each primary pipe is chosen in dependence of the flowrequirements set by the combustion chamber and the inlet manifold andair-fuel inlet pipes. In this manner, the engine can be optimised so asto provide proved performance and a high volumetric efficiency. Anotherimportant factor which is affected by the geometry of the exhaust pipesand manifold is the amount of residuals in the combustion chamber. Anoptimised design can reduce fuel consumption without leading to problemswith unstable combustion.

Preferably, the primary exhaust outlet pipes are shaped with a firstsection and a second section, the second section having a largercross-sectional area than the first section.

A further object of the invention is to provide an improved device forreducing emissions in catalytic converter exhaust systems forturbo-charged engines. This object is accomplished by means of a devicefor reducing emissions in a catalytic converter exhaust system for acombustion engine provided with a turbo-charger device, comprising anexhaust pipe connecting the engine with the catalytic converter,air/fuel mixture forming means for providing a mixture of air and fuelto the engine, a control unit adapted to control the air-fuel mixtureforming means so that a high concentration of hydrogen and othercombustible gases such as carbon monoxide and hydrocarbons are generatedin the exhaust gas during start-up of the engine, and air supply meansfor supplying secondary air downstream of at least one engine exhaustvalve during cold starting of the engine, for forming a gas mixture withthe exhaust gas Said control unit is adapted for generating an exhaustoxidation in said gas mixture and said engine is provided with anexhaust manifold, the volume of which is adapted to provide an exhaustoxidation of substantially all combustible components in said gasmixture before said gas mixture is fed to the turbo-charger device.

This object is also accomplished by means of a device for reducingemissions in a catalytic converter exhaust system for a combustionengine provided with a turbo-charger device, comprising an exhaust pipeconnecting the engine with the catalytic converter, air/fuel mixtureforming means for providing a mixture of air and fuel to the engine, acontrol unit adapted to control the air/fuel mixture forming means sothat a high concentration of hydrogen and other combustible gases suchas carbon monoxide and hydrocarbons are generated in the exhaust gasduring start-up of the engine, and air supply means for supplyingsecondary air downstream of at least one engine exhaust valve duringcold starting of the engine, for forming a gas mixture with the exhaustgas. The control unit is adapted for generating an exhaust oxidation insaid gas mixture, wherein the exhaust pipe is split to form a bypasspipe for guiding exhaust gas around the turbine and a valve is arrangedin said bypass pipe, which valve is connected to the control unit andcan be controlled to an open condition during said exhaust oxidation.

A further object of the invention is to provide an improved method forreducing emissions in catalytic converter exhaust systems. This isaccomplished by means of a method for reducing emissions in a catalyticconverter exhaust system for a combustion engine, said catalyticconverter providing reduced emissions after a light-off time has passedduring start-up of the engine, wherein the catalytic converter and theengine are connected by means of an exhaust pipe and the engine beingprovided with at least two primary exhaust outlet pipes coinciding in aplenum volume forming part of an exhaust manifold. The method comprisesthe following steps: providing a mixture of air and fuel to the enginevia air/fuel mixture forming means, controlling said air/fuel mixtureforming means so that a high concentration of hydrogen and othercombustible gases such as carbon monoxide and hydrocarbons are generatedin the exhaust gas during the start-up of the engine, supplyingsecondary air downstream of at least one engine exhaust valve duringcold starting of the engine for forming a gas mixture with said exhaustgas, and generating an oxidation of said gas mixture in the primaryexhaust outlet pipes, the exhaust manifold and the exhaust pipe, so thatheat energy generated by means of said oxidation is supplied to thecatalyst, thereby providing a reduction of the light-off time of thecatalyst.

A further object of the invention is to provide an improved method forreducing emissions in catalytic converter exhaust systems for use inturbo-charged engines. This object is accomplished by means of a methodfor reducing emissions in a catalytic converter exhaust system for acombustion engine, the catalytic converter and the engine beingconnected by means of an exhaust pipe and the engine being provided witha turbo-charger device and a plurality of exhaust outlets coinciding ina plenum volume -forming part of an exhaust manifold, said methodcomprising the following steps: providing a mixture of air and fuel tothe engine via air/fuel mixture forming means, controlling said air/fuelmixture forming means so that a high concentration of hydrogen and othercombustible gases such as carbon monoxide and hydrocarbons are generatedin the exhaust gas during the start-up of the engine, supplyingsecondary air downstream of at least one engine exhaust valve duringcold starting, for forming a gas mixture with said exhaust gas, andgenerating an oxidation of said gas mixture in said plenum volume in amanner so that an exhaust oxidation of substantially all combustiblecomponents in said gas mixture occurs before said gas mixture is fed tothe turbo device, wherein the heat energy which is generated by means ofsaid oxidation is supplied to the catalyst.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described in greater detail by way of exampleonly and with reference to particular embodiments illustrated in theannexed drawings, in which:

FIG. 1 shows schematically a diagram of a vehicle engine and an exhaustsystem comprising the present invention,

FIG. 2 shows a slightly enlarged view of an exhaust port to be used inaccordance with the invention,

FIG. 3 illustrates schematically the oxidation process in the exhaustmanifold, and

FIG. 4 illustrates schematically a second embodiment of the presentinvention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 shows in a simplified manner a system in which the deviceaccording to the present invention in incorporated. The system isarranged in connection with a combustion engine 1 which is aconventional engine of the naturally aspirated type. In a known manner,the engine 1 is supplied with an air/fuel mixture via an intake manifold2. Furthermore, the engine 1 is connected to a catalytic converter 3which preferably is in the form of a conventional three-way catalystwhich is adapted to reduce the harmful substances carbon monoxide (CO),hydrocarbon (HC) and oxides of nitrogen (NO_(x)) which are present inthe exhaust gas. The exhaust gas is discharged from the engine 1 via anexhaust pipe 4.

Downstream of the engine 1, the exhaust pipe 4 is connected to anexhaust manifold 5 through which the exhaust gas is fed. FIG. 1 depictsthree cylinder outlets in the form of primary exhaust outlet pipes (or"primary pipes") 6, 7, 8, which indicates that the engine 1 is of thetype which comprises six cylinders arranged in two banks with threecylinders each. It should be noted that FIG. 1 shows a simplified viewof the engine 1, and that certain parts such as cylinder pistons andengine valves are not shown. It should also be noted that the inventioncan be applied in engines having other cylinder configurations. Theexhaust manifold 5 is designed in a manner so that the primary pipes 6,7, 8 extend from the engine's respective cylinder to a common volume,the so-called plenum volume, which is located upstream of the exhaustpipe 4. The main purpose of the primary pipes 6, 7, 8 is to optimize theexternal gas exchange system of the engine 1.

Once the exhaust gases have passed through the catalyst 3, they aredischarged to the atmosphere through the silencer system (not shown).this is indicated by an arrow 9.

In order to obtain a suitable air/fuel mixture, the engine 1 is providedwith air/fuel mixture forming means 10 which is arranged in the intakemanifold 2. The operation of the air/fuel mixture forming means 10 iscontrolled by a electronic control unit 11 via an electrical connection12. The mixture forming means 10 comprises at least one fuel injector(not shown) and an air inlet valve (not shown). The control unit 11 isarranged to adapt the air/fuel mixture to the engine 1 in accordancewith the engine's operating conditions.

The control unit 11 is also connected to various sensors and controlfunctions of the engine 1. The control unit 11 is known per se, but hasbeen provided with certain additional control functions, as will becomeapparent below. An exhaust gas sensor, preferably in the form of anoxygen sensor 13, is arranged in the exhaust pipe 4 and provides asignal to the control unit 11, via an electrical connection 14,indicating the oxygen concentration in the exhaust gas. Furthermore, atemperature sensor 15 is arranged in connection with the catalyst 3 forproviding an indication of the temperature of the catalyst 3 to thecontrol unit 11 via a further electrical connection 16. It should benoted that information in the control unit 11 regarding the temperatureof the catalyst 3 can also be provided by means of a software modelwhich provides a measure of the temperature as a function of one or moreof various operational parameters of the engine, for example thetemperature of the cooling water and the ambient temperature. By meansof such parameters, a value corresponding to an "expected" catalysttemperature can be calculated in the control unit 11. This value can beused for controlling the system in accordance with principles to beexplained below. It should be noted that if the temperature of thecatalyst 3 is to be calculated by means of a software model, no separatetemperature sensor 15 is required.

By means of additional (not shown) sensors, e.g. a sensor for thecooling water, sensors for the temperature of the engine and the ambientatmosphere, an engine speed sensor, an engine crankshaft positionsensor, an air mass flow meter and a throttle angle indicator, thecontrol unit 11 is supplied with information regarding the engineoperation. In a manner which is known, the control unit 11 controls theair/fuel mixture according to any given operating conditions of theengine.

According to the invention, the control unit 11 is adapted to controlthe operation of the engine 1 during cold starting in a manner so as toobtain a relatively high concentration of hydrogen in the exhaust gas.In this regard, the air/fuel mixture to the engine 1 in controlled so asto provide a stoichiometric air/fuel ratio corresponding toapproximately λ=0.6-0.8, ices the air/fuel mixture to the engine isgiven an excess of fuel which, according to the known principles whichhave been described above, generates a certain amount of hydrogen andcarbon monoxide in the exhaust gas.

In addition to the excess of fuel which is present in the air/fuelmixture to be delivered to the engine, the engine speed and the ignitiontiming are factors which also determine the amount of hydrogengenerated. However, the hydrogen concentration in the exhaust gas isessentially determined by the stoichiometric fuel excess. The enrichedair/fuel mixture which is present during this cold start operationprovides a hydrogen concentration which preferably amounts toapproximately 4-6% of the total gas volume in the exhaust gas.

Furthermore, and according to the invention, additional (or secondary)air should be added to the exhaust gas, thereby producing a gas mixturecomprising exhaust gas and secondary air. According to a firstembodiment of the invention, this secondary air is supplied by means ofan air supply system in the form of an air pump 17, the output of whichis connected via an air duct 18 to the exhaust manifold 5. The air pump17, which is connected to the control unit 11 via an electricalconnection 19, is preferably operated so as to provide secondary airduring the time period during which an excess of hydrogen and carbonmonoxide in the exhaust gas is generated, i.e. the time period up to adesired level of purification of harmful components in the exhaust gasby the catalyst 3 during cold starting. The desired level may or may notcorrespond to the 50% level described above.

The air duct 18 is designed with a number of branches 20, 21, 22, theoutlets of which terminate in a corresponding primary pipe 6, 7 and 8,respectively. It should be noted that the number of branches of the airduct 18 preferably corresponds to the number of primary pipes of theengine 1.

The air pump 17 is adapted to be controlled by the control unit 11 toprovide a flow of secondary air from the atmosphere, which flow isforced in through an inlet duct 23 forming part of the air pump 17.Furthermore, the air pump 17 is designed in a manner so as to provide anair flow of sufficient pressure to assure the desired air/fuel ratio inthe exhaust gas mixture.

Furthermore, and according to known principles, the concentration ofhydrogen in the exhaust gases in combination with the addition ofsecondary air to the exhaust gas provides an increased oxidation ofcombustible components in the exhaust gas. More specifically, thehydrogen, carbon monoxide and hydrocarbons in the exhaust gas will reactwith the oxygen being supplied by the secondary air if a certainthreshold temperature T_(t) in the gas mixture is reached. This timeaveraged threshold temperature T_(t) is approximately 300-450° C.

The exhaust gas system according to the invention can also be providedwith a so-called hydrocarbon adsorber 24, which is arranged upstream ofthe catalyst 3 or, alternatively, integrated within the catalyst 3.During operation of the engine 1, the hydrocarbon adsorber 24 adsorbshydrocarbon compounds present in the exhaust. When the temperature ofthe exhaust gas, and thus also of the hydrocarbon adsorber 24, exceeds acertain threshold value, the hydrocarbon adsorber 24 will desorb thehydrocarbons which so far have been collected therein. This thresholdvalue, which reflects the equilibrium state is usually below thecatalyst light-off temperature. A relatively slow increase in exhausttemperature, which is usually the case in conventional engines,therefore results in a limited amount of hydrocarbons remaining adsorbeduntil the catalyst has reached the light-off temperature. However, therapid increase in exhaust temperature due to the invention reduces theamount desorbed from the hydrocarbon adsorber 24 before the catalyst 3has reached its light-off temperature.

FIG. 2 illustrates, on a larger scaler a perspective view of a primarypipe 6, i.e. one of the three cylinder outlets shown in FIG. 1. Theexhaust gases of the engine 1 are discharged via two exhaust valves 25,26. For reasons of clarity, FIG. 2 also depicts a piston 27 forming partof the engine. It will be readily apparent for those skilled in the artthat the engine may comprise for example six cylinders, each one havinga primary pipe with the same design as that which is shown in FIG. 2.

Furthermore, the air duct branch 20 is arranged in the primary pipe 6 ina manner so that it terminates in close connection to the exhaust valves25, 26. Preferably, the air duct branch 20 is arranged so that theinjected secondary air is directed towards a point located substantiallyhalf-way between the valves 25, 26. The air duct branch 20 is formed asa moulded duct in the cylinder head of the engine. It should be notedthat the air supply system may alternatively be designed with one airduct branch for each exhaust valve.

In accordance with the invention, the primary pipe 6 (as well as theother primary pipes 7 and 8 shown in FIG. 1) is formed by a firsttubular section 28 and a second tubular section 29. The second section29 has a cross-sectional area which exceeds that of the first section28. Preferably, the cross-sectional area of the second section 29 isapproximately two to five times the cross-sectional area of the firstsection 28, as regarded perpendicular to the direction of the exhaustgas flaw The sections 28, 29 are preferably circular, but other designsare also possible within the scope of the invention, for examplerectangular or elliptical. The transition from the first section 28 tothe second section 29 is preferably arranged aligned (i.e. flush) withthe top side of the engine's 1 cylinder head.

The operation of the invention will now be described. The engine 1operates according to the four-stroke cycle, which means that theexhaust valves 25, 26 are closed during the compression and expansionphases of the engine 1. During this period, the first section 28 issubstantially filled with secondary air via the air duct branch 20.Also, the second section 29 will normally be at least partly filled withsecondary air. When the valves 25, 26 are opened, the exhaust gas isdischarged and forms the above-described gas mixture together with thesecondary air. The operation of the air pump 17 for injection ofsecondary air is controlled by the control unit 11 so as to provide anoxidation of said gas mixture. The system is preferably operated so asto provide an air/fuel ratio of the gas mixture of approximatelyλ=1.0-1.3.

The oxidation process in the primary pipe 6 is schematically illustratedin FIG. 3, which shows the temperature T of said gas mixture (i.e. themixture of the secondary air and the exhaust gas) as a function of timet. It should be noted that as time t elapses, the gas mixture flowsalong the pipe 6. Consequently, the temperature T at a certain point intime is not related to a specific position in the pipe 6.

When the exhaust valves 25, 26 are opened (i.e. t=0), the rapid mixingof the exhaust gas and the secondary air causes the gas mixture to losesome of its heat energy, consequently reducing its temperature T (FIG.37 step 30). However, the oxidation of the gas mixture which then occursafter a short time period will increase the temperature T (step 31). Theposition of the second section 29, i.e. the length of the first section29, can be chosen at a distance which is further away from the exhaustvalves 25, 26, due to the fact that there is an increase of thetemperature in the first section 28. Furthermore, the fact that the gasmixture flows along the first section 28 will reduce the gas temperatureT (stop 32) due to the cooling effect which the inner walls of the firstsection 28 imposes on the gas mixture.

When the gas mixture "pulse" reaches the second section 29, thetransition from the first section 28 to the second section 29 willresult in a new reduction of the temperature (step 33). This is due tothe fact that the gas mixture which flows into the second section 29will be mixed with a relatively high gas volume (i.e. a mixture ofexhaust gas and secondary air) which was fed to the second section 29during the previous operation phase. The volume of this "old" gasmixture depends on the increase in volume from the first section 28 tothe second section 29, i.e. the relationship between theircross-sectional areas.

The expanding volume in the second section 29 provides access to oxygenin the "old" gas mixture so that the exhaust oxidation can be furtherdeveloped. This mixing is aided by means of turbulence which isgenerated in the second section 29. Again, this results In an increaseof the temperature T of the gas mixture (step 34). The gas mixture thenflows along the second section 29, resulting once more in a reduction ofthe temperature T, mainly due to the cooling effect of the inner wallsof the second section 29 (step 35). Furthermore, the gas mixture reachesthe plenum volume (cf. FIG. 1) where all the primary pipes 6, 7, 8coincide. The temperature T decreases in the plenum volume (step 36),once again due to the increase in volume. Finally, the exhaust oxidationcontinues in the plenum volume and along the exhaust pipe 4 (step 37).The heat energy of the gas mixture is transferred to the catalyst 3 (andto the hydrocarbon adsorber 24, if such a device is used), which willbecome rapidly heated.

An advantage of the invention is that for each oxidation stage from theexhaust valves 25, 26 to the exhaust pipe 4, the critical temperature atwhich oxidation can occur is always exceeded, due to the fact thatmixing with air occurs at each stage. In this manner, there is no needto use a separate ignition device in order to ignite the gas mixture.

A further advantage of the invention is that a large part of the exhaustoxidation takes place close to the catalyst. In this way, very lowlosses in heat energy will occur until the gas mixture reaches thecatalyst.

The on and off switching of the air pump 17 for injection of secondaryair is controlled by the control unit 11 s0 as to provide an oxidationof said gas mixture. The system is preferably operated so as to providean air/fuel ratio of approximately λ=1.0-1.3. In this manner, thethreshold temperature T_(t) can be exceeded so that oxidation occurs inthree different main stages, i.e. immediately downstream of the exhaustvalves 25, 26, in the transition between the first section 28 and thesecond section 29 of the primary pipe 6, and in the plenum volume of theexhaust manifold 5. Experiments have shown that the system according tothe invention provides a time taken for the light-off temperature of thecatalyst 3 to be reached which is approximately equal to 5-10 seconds.

The oxidation occurs in a mater so that all the necessary conditions(for example as regards temperature) are fulfilled. The first stage ofthe oxidation provides a possibility to arrange the subsequent stages ofthe oxidation at a greater distance from the exhaust valves thanotherwise would have been possible. In this manner, it is possible tooptimize the external gas exchange of the engine by means of the higherdegrees of freedom which this "multi-stage" process provides.

It should be noted that due to the fact that the control unit 11contains information regarding the temperature of the catalyst 3, thecontrol unit 11 is adapted to control the supply of secondary air andthe generation of hydrogen in the exhaust gas depending on thetemperature of the catalyst. This can be accomplished through the factthat the temperature sensor 15 is connected to the control unit 11. Theoperation of the invention can be controlled in dependence of the valueof the temperature. For example, the generation of hydrogen and thesupply of secondary air can be switched off when the catalyst'spurification has reached a certain level. This level can be calculatedfrom a temperature value. Also, the control unit 11 may be arranged toinitiate the generation of hydrogen and the supply of secondary air ifthe temperature value is lower than a certain limit when the engine isstarted. According to an alternative embodiment, the control unit 11calculates an "expected" catalyst temperature from values of the ambienttemperature, the temperature of the cooling water of the engine or otheravailable engine parameters. The oxidation process is continued until adesired level of conversion of harmful components in the exhaust gas bythe catalyst 3 is reached, as estimated from the catalyst temperature. Acertain temperature value may be used for starting the oxidationprocess, and another temperature value may be used for terminating theoxidation process.

It should be noted that in accordance with the invention the increase intemperature is as rapid that a substantial part of the hydrocarboncomponents will not desorb from the hydrocarbon trap 24, in spite of thefact that this would normally had taken place (during equilibrium).

According to a further embodiment, the control unit 11 can be adapted tocontrol the oxidation in a manner so that essentially no oxidationoccurs until the exhaust gas reaches the catalyst- This is achieved byadjusting the spark retardation of the engine to a value whichessentially corresponds to normal idling, i.e. the spark timing is notdelayed. The advantage with this operation is that very low losses ofheat energy occur in the manifold.

FIG. 4 illustrates schematically a system comprising a second embodimentof the present invention, which is arranged in connection with acombustion engine 38 of the type which is provided with a turbo device39. The turbo device 39 is previously known and comprises a turbine 40through which the exhaust gas is fed via an exhaust pipe 41. The exhaustgas is further fed to a catalyst 42, preferably a conventional three-waycatalyst.

The turbine 40 Is arranged on a shaft 43 which is common with acompressor impeller 44, which in turn forces air from the atmosphere viaan input duct 45 into the engine 38. In a conventional manner, thisinput air is mixed with fuel in air/fuel mixture forming means 46 of thetype which is generally similar to that which has been described above.The mixture forming means 46 is controlled by an electronic control unit47 via an electrical connection 48. The mixture forming means 46comprises at least one fuel injector (not shown) and an air inlet valve(not shown) and can be controlled so as to adapt the air-fuel mixture tothe engine 38 in accordance with the operating conditions.

The control unit 47 is also connected to various sensors and controlfunctions of the engine 38. This is generally similar to that which hasbeen described with reference to FIG. 1. In particular, the systemcomprises an oxygen sensor 49 which is connected to the control unit 47via an electrical connection 50, and a catalyst temperature sensor 51which is connected to the control unit 47 via a further electricalconnection 52.

The control unit 47 is adapted to control the operation of the engine 38during cold starting in a manner so as to obtain a relatively highconcentration of hydrogen in the exhaust gas. Furthermore, the controlunit 47 is adapted to control an air supply system in the for of an airpump 53 having an input air duct 54 for air from the atmosphere and anoutput air duct 55 connected to the exhaust manifold 56. The air pump53, which is connected to the control unit 47 via an electricalconnection 57, is preferably operated so as to provide secondary airduring the time period during which an excess of hydrogen in the exhaustgas is generated. Since the embodiment illustrated in FIG. 4 is oneincluding a turbo device 39, the exhaust pressure is increased ascompared to an engine without a turbo device. Consequently, the air pump53 must be adapted to supply an air pressure which is high enough toforce air into the exhaust manifold 56 In spite of the high pressurewhich prevails therein.

The engine 38 is illustrated with an exhaust manifold 56 having threecylinder outlets 52, 59, 60, indicating that the engine 38 is of thesix-cylinder type. In order to provide a quick response of the turbodevice 39 during operation, the volume of the exhaust gas manifold 56 ispreferably kept as low as possible while still keeping the residencetime of exhaust gases upstream of the turbine 40 sufficiently long so asto provide an exhaust oxidation of substantially all combustiblecomponents in the exhaust gas before the gas mixture is fed to theturbine 40.

The air duct 55 is designed with three branches 61, 62, 63, whichcorrespond to each of the cylinder outlets 58, 59, 60. According to analternative embodiment, the cylinder outlets 58, 59, 60 can be providedwith primary pipes of the type which has been described above.

In accordance with the embodiment, and during cold starting of theengine 38, secondary air is mixed with the exhaust gas. This generatesan exhaust oxidation, which increases the heat energy in the exhaustsystem.

The turbo device 39 is provided with a waste gate valve (not shown)which is arranged in connection with the turbine 40. The waste gatevalve can open to provide a bypass passage in case the exhaust gaspressure should be too high. Preferably, the waste gate valve is of thetype which is electrically controllable. However, a pressure-controlledwaste gate valve can also be used. The waste gate valve is connected tothe control unit 47 via a further electrical connection 64. During coldstarting, i.e. when the control unit 47 has determined that exhaustoxidation should be initiated, the waste gate valve can be controlled toassume its open or partially open condition in order to control thepressure in the exhaust manifold, which in turn makes it possible tooptimise the oxidation.

Furthermore, the exhaust pipe 41 is preferably split into a bypass pipe65, which is provided with a controllable valve 66. The valve 66 isconnected to the control unit 47 via an electrical connection 67. Bymeans of this arrangement, the control unit 47 is adapted to control theoperation of the valve 66 for bypassing the exhaust gas around theturbine 40. This is particularly advantageous during cold starting sinceexhaust oxidation can then occur in the bypass pipe 65 and furtherdownstream in the exhaust system. The controllable valve 66 shown inFIG. 4 is arranged at the upstream end of the bypass pipe 65.Alternatively, the valve 66 may instead be arranged at the downstreamend of the bypass pipe 65. Furthermore, the valve 66 may be integrallydesigned with the above-mentioned waste-gate valve, thereby forming onesingle controllable valve which can be used for directing the exhaustgas.

The bypass pipe 65 is preferably designed with relatively largedimensions. In this manner, the volume of the exhaust manifold 56 can berelatively small, which in turn provides a quicker response of the turbooperation. Furthermore, the fact that the manifold 56 is of smalldimensions constitutes an advantage, since it makes the engine systemmore compact.

According to an embodiment which is not shown in the figures, theprimary pipes (not shown) and/or the bypass pipe 65 can be designed astwo or more sections of different cross-sectional areas, similar to thatillustrated in FIG. 2.

Furthermore, the system illustrated in FIG. 4 is preferably providedwith a hydrocarbon adsorber 68 arranged upstream of the catalyst 42. Theoperation of this hydrocarbon adsorber is the same as that which hasbeen described above.

Whilst the invention has been described above with respect to certainpreferred embodiments thereof, the invention is not limited to these butmay be varied widely within the scope of the appended claims. Forexample, the primary pipes mentioned above can be provided with morethan two sections having different cross-sectional areas.

Furthermore, according to an alternative embodiment, the above-mentionedhydrocarbon adsorber can be omitted. Also, according to yet anotheralternative embodiment, the bypass pipe 64 with its valve 65 (cf. FIG.4) can be omitted from the exhaust system. Finally, in the case wherethe bypass pipe 64 is used, the cylinder outlets can be provided withprimary pipes of the same types as illustrated in FIG. 1.

What is claimed is:
 1. A device for reducing emissions in a catalytic converter exhaust system for a combustion engine, said catalytic converter providing reduced emissions after a light-off time has passed during start-up of the engine, said device comprising:an exhaust pipe connecting the engine with the catalytic converter, an air/fuel mixture forming means for providing a mixture of air and fuel to the engine, a control unit adapted for controlling the air-fuel mixture forming means so that a high concentration of hydrogen and other combustible gasses are generated in the exhaust gas during start-up of the engine, and an air supply means for supplying secondary air downstream of at lease one engine exhaust valve during cold starting of the engine, for forming a gas mixture with the exhaust gas, said engine being provided with a plurality of primary exhaust outlet pipes, wherein said primary exhaust outlet pipes are shaped with a first section and a second section, the second section having a larger cross-sectional area than the first section, said second section positioned downstream of said first section with respect to the direction of flow of the exhaust gas, and wherein said control unit is adapted to control said generation of hydrogen and the operation of said air supply means, thereby generating an exhaust oxidation in said gas mixture, so that heat energy generated during the oxidation is supplied to the catalyst, thereby providing a reduction to the light-off time of the catalyst.
 2. Device according to claim 1, wherein said primary exhaust outlet pipes are shaped with a first section and a second section, the second section having a larger cross-sectional area than the first section.
 3. Device according to claim 1, wherein the cross-sectional area of the second section is approximately two to five times the cross-sectional area of the first section, as regarded perpendicular to the exhaust flow direction.
 4. Device according to claim 3, the engine being provided with a cylinder head, wherein the upstream end of the second section is located aligned with the top side of said cylinder head.
 5. Device according to claim 4, wherein the control unit is adapted to control the generation of hydrogen and the operation of said air supply means depending on the temperature of said catalyst.
 6. Device according to claim 5, wherein the catalyst is equipped with a temperature sensor.
 7. Device according to claim 5, wherein the control unit is adapted for calculating an approximate value of said temperature depending on operational parameters of the engine.
 8. Device according to claim 1, wherein a hydrocarbon adsorber is arranged upstream of the catalyst or is integrated within the catalyst.
 9. The device of claim 1, wherein said other combustible gases are selected from the group consisting of carbon monoxide and hydrocarbons.
 10. Method for reducing emissions in a catalytic converter exhaust system for a combustion engine, said catalytic converter providing reduced emissions after a light-off time has passed during start-up of the engine, wherein the catalytic converter and the engine are connected by an exhaust pipe and the engine being provided with at least two primary exhaust outlet pipes coinciding in a plenum volume forming part of an exhaust manifold, wherein said primary exhaust outlet pipes are shaped with a first section and a second section, the second section having a larger cross-sectional area than the first section, said second section positioned downstream of said first section with respect to the direction of flow of the exhaust gas, said method comprising the following steps:providing a mixture of air and fuel to the engine via air/fuel mixture forming means, controlling said air/fuel mixture forming means so that a high concentration of hydrogen and other combustible gases are generated in the exhaust gas during the start-up of the engine, supplying secondary air downstream of at least one engine exhaust valve during cold starting of the engine for forming a gas mixture with said exhaust gas, and generating an oxidation of said gas mixture in the primary exhaust outlet pipes, the exhaust manifold and the exhaust pipe, so that heat energy generated by means of said oxidation is supplied to the catalyst, thereby providing a reduction of the light-off time of the catalyst.
 11. Method according to claim 10, wherein said oxidation is controlled in the following steps:generating a first oxidation stage in said gas mixture essentially immediately downstream of said at least one exhaust valve, mixing said gas mixture with further air, and generating a second oxidation stage in said gas mixture after said nixing with further air.
 12. Method according to claim 11, wherein the generation of hydrogen and the operation of said air supply is made depending on the temperature of said catalyst.
 13. The method of claim 10, wherein said other combustible gases are selected from the group consisting of carbon monoxide and hydrocarbons. 